The present invention is in the field of organic color pigments. It is known that organic pigments, especially azo pigments, are precipitated from the synthesis solution as small insoluble particles (primary crystallites) which require an aftertreatment (finish). In the course of this treatment, it is necessary to modify physical properties, such as crystal form, crystal size and crystal quality, and also particle size distribution, toward a desired optimum. If a crude pigment presscake is in fact dried directly after synthesis and washing, there is often considerable congregation of the primary particles to form agglomerates and aggregates. This leads to pigments with hard grains, weak in color and difficult to disperse, which are often impossible to bring into a technically usable form, even by milling. Polycyclic pigments are precipitated from the synthesis solution usually as coarsely crystalline crude pigments, which need to be subsequently finely divided by means of appropriate techniques, such as milling, for example. In the majority of cases, the prepigments thus obtained also require an aftertreatment in order to obtain the desired physical properties.
The standard pigment finish is a thermal aftertreatment in which improved crystal formation is achieved by heating the crude pigment suspension or the pigment presscake, washed to remove salts, isolated and pasted up again, in water and/or organic solvents. In this process, the ultrafine fraction, which is responsible in particular for the agglomeration tendency of the pigments, is reduced and, consequently, a narrower particle size distribution is obtained. In organic solvents, pigments of particularly low solubility are aftertreated at temperatures of from 80 to 150xc2x0 C. Solvents used for this purpose include, for example, alcohols, glacial acetic acid, chlorobenzene, o-dichlorobenzene, and dimethylformamide. The finishing techniques which have been customary to date are expensive in terms of apparatus and energy, since heating and the distillative removal of the solvent are often conducted under pressure. Since the majority of organic solvents are flammable, appropriate measures must be taken for plant safety.
The object of the present invention was to provide an appropriate finishing process for organic pigments which is superior to the hitherto customary solvent finish in terms of safety, environmental compatibility, and resource consumption.
It has been found that liquid or supercritical carbon dioxide is, surprisingly, a suitable finishing medium for organic pigments.
The present invention accordingly provides a process for aftertreating organic pigments, which comprises causing ground or unground arrange crude organic pigment and liquid or supercritical carbon dioxide to act on one another.
The procedure here is judiciously to filter the crude pigment suspension which is present following pigment synthesis or following a fine division, e.g., by grinding, to wash the solid filter product, to dry the washed product to give the crude pigment powder, and to add liquid or supercritical CO2. Instead of the dried crude pigment powder it is also possible to use a water-moist or solvent-moist crude pigment, e.g., a filter cake or presscake.
The aftertreatment of the invention is judiciously conducted in a heatable autoclave with stirring apparatus. It is not necessary to pump off the residual air still present in the autoclave. Carbon dioxide may be added in the form of dry ice or by pumping in gaseous or liquid CO2, the amount being calculated such that the pressure which is established at the desired temperature is a pressure at which CO2 is in the supercritical or liquid state. The preferred temperature range is situated between 31.2xc2x0 C. and 250xc2x0 C., with particular preference between 50 and 200xc2x0 C., in particular between 80 and 180xc2x0 C., for supercritical CO2. The pressure which is established is from 73.8 bar to preferably 400 bar, with particular preference between 75 and 300 bar, in particular between 80 and 250 bar. For liquid CO2, the preferred temperature range is from 10 to 31.2xc2x0 C., in particular from 20 to 31xc2x0 C. Relative to the weight of the crude pigment, the CO2 is used judiciously in an amount (CO2: pigment) of from (0.2:1) to (200:1), preferably from (0.5:1) to (10:1).
The duration of the aftertreatment may vary within wide limits, with from 10 minutes to 10 hours being judicious and from 0.5 to 5 hours being preferred. Subsequently, the autoclave is let down to atmospheric pressure and emptied.
It may be advantageous to conduct the aftertreatment of the invention in the presence of water or organic solvents, such as hydrocarbons, alcohols, ethers, amines, carboxylic acids, carboxylic esters or carboxamides, for example, such as N-methylpyrrolidone, for example. In this case, the amount of water or solvent added should preferably be at most that which is soluble in carbon dioxide under the prevailing conditions. Thus, for example, at 75xc2x0 C. and 200 bar, about 5 g of water are soluble per kilogram of CO2.
The aftertreatment of the invention may be performed with all organic color pigments, such as azo pigments and polycyclic pigments. Azo pigments may be monoazo, disazo, disazocondensation, naphthol, or metal complex pigments.
Particularly appropriate azo pigments are C.I. Pigment Yellow 16, 32, 83, 97, 120, 151, 154, 155, 175, 180, 181, 191, 194, 213, Pigment Orange 34, 36, 38, 62, 72, 74, Pigment Red 53:2, 112, 122, 137, 144, 170, 171, 175, 176, 185, 187, 188, 208, 214, 242, 247, 253; Pigment Violet 32; Pigment Brown 25.
Polycyclic pigments may be, for example, isoindolinone, isoindoline, anthanthrone, thioindigo, quinophthalone, anthraquinone, dioxazine, phthalocyanine, quinacridone, perylene, perinone, diketopyrrolopyrrole, thiazoindigo and azomethine pigments, especially Pigment Violet 19, 23, Pigment Blue 15, Pigment Green 7, 36, 37, Pigment Red 122, Pigment Yellow 139.
It has been found that the aftertreatment of the invention may replace the aqueous or solvent finish which has been customary to date. Furthermore, with certain pigments, depending on temperature, pressure, treatment period and addition of water, there may surprisingly also be a change in other physical properties, such as the crystal modification or the ratio of the crystal modifications formed, for example.